DESCRIPTION (Investigator's Abstract): Fish oils, a rich source of long chain polyunsaturated omega-3 fatty acids, have been linked through epidemiologic studies to the prevention of a number of important human afflictions including cancer, arthritis and heart disease. Although the mechanism of action for these compounds is unknown, one possibility is that they are incorporated into phospholipids and they affect some important membrane properties. The process of aging involves deterioration of many physiological functions including bioenergetic events associated with mitochondria. The age related decrease in P/O ratios and state three respiration is known to be linked to changes in mitochondrial lipid composition. Old mitochondria have less unsaturated fatty acids, lower membrane fluidity and enhanced membrane permeability when compared to younger mitochondria. Upon incorporating fish oils into animal diets, mitochondrial levels of omega-3 fatty acids are known to increase. This project will test whether incorporation of fish oils into the diets of mice can prevent or even reverse the age dependent decrease in mitochondrial function. Mouse diets will be supplemented with either hydrogenated cotton seed oil (rich in saturated fatty acids) or menhaden oil (a source of omega-3 fatty acids). Mitochondrial properties to be followed as a function of age will include lipid analysis, respiratory control index and P/O ratios, membrane fluidity, lipid peroxidation, permeability to protons and electrical conductivity.